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36 MAYLAND ET AL. calculated EF of the 2µm MMAD particle was 2 times that of the 50-µm particle. In another study conducted in the USA (Campbell et al., 1978), the EF for Se in the fine particles compared to larger particles was 9.9, illustrat- ing that some variation occurs between coal plants or in sampling and ana- lytical techniques. High combustion temperatures volatilize some of the Se in the coal, which then leaves the furnace in the gaseous phase. Fly ash and gases begin to condense as they move through the particle scrubbers and ascend the stacks. Most of the larger particles and the Se associated with them are trapped by the scrubbers. However, the remaining Se probably condenses on the smaller particles that escape the stack and enter the atmosphere as aerosols. The degree of preferential enrichment may be a function of the individual power plant and current operating conditions. The amount of Se leaving the stack will be a function of the Se content of the coal, particle size, combustion temperature, efficiency of particle scrubbers, temperature of flue-gases, and exit velocity. Another source of aerosols that can enter the atmosphere and contrib- ute to global cycling of Se is the incineration of refuse. Nationally we gener- ate 16 x 1010 kg of municipal refuse annually in the USA (Lemley, 1987). While incineration of wastes would reduce this mass, it would also be a source of aerosol and ash residues. In the United Kingdom, municipal refuse in- cineration consumed 3 x 109 kg of material annually generating 1 x 109 kg ash residue (Wadge et al., 1986). Unlike coal-fired power plants, more bottom ash is produced than fly ash. Refuse incinerators operate at lower temperatures (800-1100°C) compared to coal furnaces that operate at near- ly 1500°C. Coal fly ash is composed of cenospheres of <20 µm MMAD, whereas refuse fly ash consisted mostly of amorphous particles of < 50 µm MMAD. Wadge et al. (1986) reported that refuse fly ash from an incinerator located in the United Kingdom contained 2.5 to 6.6 mg Se/kg with a mean value of 4.4. The Se in this refuse fly ash had an EF of 93 compared with an EF of 98 reported for refuse fly ash from an incinerator in the USA (Green- berg et al., 1978a). These EF values are similar to those given for coal-derived fly ash of 84 and 132 given above. Greenberg et al. (1978b) reported an EF of 4500 for Se in aerosols above a refuse incinerator in the USA, possibly reflecting the larger particle sizes and cooler incinerator temperatures when compared with data for coal furnaces (Andren et al., 1975). Although the EF values of Se are similar for the two refuse incinerators, one should not assume that all municipal refuse is alike or their Se concentrations the same. Measurable differences may exist among Se concentrations in refuse from rural vs. industrial vs. metropolitan areas. Chiou and Manuel (1986) collected atmospheric particulates at Rolla, MO, using ahigh-volume particle fractionating cascade impactor. This sys- tem gave five sized fractions with a range of 0.01 to > 7.0 µm MMAD parti- cle. They measured 3.6 ± 0.8 µg Se/m3 compared with a range of 2.1 to > 7µg/m3 measured elsewhere in the USA (Ondov et a1., 1982; Pillay et al.,